1. Technical Field
The present invention relates generally to a system and method for stabilizing and immobilizing a plurality of adjacent bone segments. A typical, but non-limiting application of this system is in the field of posterior spinal fixation wherein a single system can be readily adapted for simple stabilization of adjacent vertebra or to effect the controlled angulation needed for reduction and distraction.
2. Description of the Prior Art
Posterior spinal stabilization systems are typically used for the treatment of conditions involving vertebral displacement such as kyphosis, spondylolisthesis and rotation; segmental instability, such as disc degeneration and fracture caused by disease, trauma, and congenital defects; and tumor diseases. The preferred method of spinal stabilization is immobilization of the affected joint(s) by surgical fusion, or arthrodesis. In order to permit fusion of adjacent vertebrae, immediate immobilization of any affected joints is necessary, which is preferably performed by internal fixation.
Internal fixation refers to methods of stabilization which are wholly internal to the patient and which include commonly known appliances such as bone plates or rods, hooks, and screws. Internal fixation is now the favored means of immobilization since the lack of external components allows the patient greater freedom of movement and reduces the possibility of infections. Prior art devices for internal posterior spinal fixation generally incorporate one of two basic design developments, top loading transpedicular plate systems and side loading elongated rod distraction/compression fixation systems.
Transpedicular plate systems comprise one or more parallel elongated plates having spaced apertures or longitudinal slots through which pedicle screws may be received and secured by appropriate fastening devices. The pedicles are a favored area of attachment upon the vertebrae since they offer an area of sufficient size, depth, and strength to hold the fixation device. Transpedicular screw and plate systems rely on a screw threaded into a reamed canal generally positioned perpendicular to the longitudinal axis of the spine and horizontal or parallel to the anterior/posterior plane of the vertebral body. Methyl methacrylate is sometimes used to secure the screw in the canal, particularly if the screw loosens or osteoporosis is a problem. The screws engage a plate which has been bent to conform to the normal curvature of the spine or to the points of desired reduction.
A transpedicular screw and plate system developed by Roy-Camille utilizes plates having spaced, predrilled holes. The pedicles are located, reamed, tapped, and two screws are inserted into each vertebral body. Corticocancellous bone graft is placed in the lateral gutter over the transverse processes. The upper portion of each screw is threaded and extends through a respective hole in the plate, being held there by a corresponding nut. This simple system was "top loading" in that it allows the surgeon to tighten all appliance connections through the center of the plate and from the top. However, although the Roy-Camille system immobilized or fixated the spinal segments, it provided minimal ability to apply any corrective forces thereto and forced the screw into a non-anatomic position to conform to the preformed holes.
A later top loading system, developed by Steffee, is known as the Variable Spinal Plate (VSP) system. Using the Steffee system, screws are inserted into the pedicles as described above. Slotted plates are contoured to a desired sagittal curvature and placed over the screws, following which tapered nuts are threaded onto the screws. The screws are then tightened bilaterally until the plate is locked between two nuts. The VSP system allows some limited application of corrective forces to the vertebrae along both axial and vertical directions but does not permit controlled angular positioning of the pedicle screws. Thus one drawback of the VSP system is that it is limited to fixation in two planes.
U.S. Pat. No. 5,084,049 to Asher et al. discloses a transpedicular plate system which permits fixation in three planes by using a transverse connector member to maintain the two slotted plates in a selected spaced relationship. Clamps at opposite ends of the transverse connector secure respective slotted plates at a desired angular orientation with respect to the horizontal plane. The Asher system, however, does not permit the surgeon to vary or control the angulation of the screws segment by segment, and it is not top loading in that the transverse connector requires tightening of set screws from the side.
A significant drawback of the above described slotted plate systems is that the risk of pedicle screw breakage is increased because of the need to conform the plates to sagittal curvature by bending. Specifically, to the extent that the angle of a screw varies from a perpendicular relationship to the portion of the plate to which it is affixed, the screw will be forced to either bend or seat in a position which generates constant torque. Variance from a perpendicular relationship may thus result in constant unidirectional torque of the screw against one wall of the pedicle, which may in turn cause weakening of the screw, erosion of the bone or undesirable shifting of the vertebrae-to-adjacent-vertebrae angular relationship.
Where treatment requires the introduction of corrective forces to the vertebrae at various transverse angles to the horizontal plane (i.e., reduction/distraction), present systems employ a system of elongated rods having circular cross sections. The circular cross section of the rods permits rotation of appliances which secure bone hooks or screws to the rod. Once the surgeon determines how much corrective force he wishes to apply to each vertebrae, the elongated rods of these systems are bent to achieve the desired spacing between the rod and each of the given vertebrae to be corrected. Combinations of hooks (laminar, pedicle, and transverse), sacral attachments (screws or rod extensions), or supporting wires are then connected to the rods at desired locations using the rotatable appliances. Examples of such systems include the Harrington distraction system sold by Zimmer USA, Inc., the Keene system shown in U.S. Pat. No. 4,269,178, and the Lewis-Greenlaw system illustrated in U.S. Pat. No. 4,085,744.
These systems are side loading in that the hooks and screws which are secured to the rod must be fastened along lines which are spaced from the longitudinal axis of the rod. Because of this spacing, the side-loading rod systems are more difficult to install than top-loading, slotted plate systems, especially where deep, angled incisions are involved (such as in fixation of the low lumbar spine).
Another drawback of the elongated rod systems is their susceptibility to rotational misalignment. Specifically, the aforementioned spacing between the longitudinal axis of the rod and the post attachment point creates a considerable risk of in situ rotational displacement because the reactive forces of the posts are directed about the center axis of the rods. The greater the spacing between the post connection point and the rod center axis, the greater the risk of rotational displacement of the rod.
The recently developed AMSET.TM. R-F system also utilizes threaded rods to effect segment by segment angular positioning of pedicle screws. Unlike the elongated rod systems discussed above, however, the R-F system also provides some of the benefits associated with the top loading slotted plate systems. To effect angular positioning relative to a vertical plane transverse to the longitudinal axis of the rod, the R-F system employs screws which have a unique U-shaped head design, wherein the threaded portion of the screw is available at angles of 0.degree., 5.degree., 10.degree. and 15.degree. relative to the head. The head defines a channel dimensioned to receive the threaded rod. Rotation of the head relative to the rod effects angular positioning relative to a horizontal plane. A pair of traction nuts engage respective faces of the head to maintain the screw in the desired orientation. When only simple fixation is required, the R-F system utilizes a specially machined block having a smooth vertical bore for receiving the upper threaded portion of a pedicle screw and a horizontal bore for sliding movement along the rod. Blocks are slid into desired positions along the rod to line up with inserted pedicle screws and are locked in place by pairs of traction screws. When the rod is installed, the respective blocks receive corresponding screws and nuts are tightened thereon to lock the rod into position.
Although the R-F system does provide an elongated rod system capable of providing both top loading simple fixation and selected segment by segment angular correction, it does not provide all of the benefits of the slotted plate system. Specifically, when used to provide simple fixation, it still suffers from the risk of unbalanced reactive forces as a result of the spacing between the axis of the rod and the post attachment points.
Thus, it should be apparent that in exchange for their ability to allow graded, angular movement at each segment, the elongated rod systems sacrifice the simplicity and rotational stability of slotted rod systems. Accordingly, unless the treatment requires segment by segment angular positioning of the screws, it will generally be more appropriate to use a slotted plate system.
Heretofore, the distinct advantages of the slotted plate systems and the elongate rod systems have required the surgeon to learn how to use a form of each system and the hospital to buy or have access to a variety of different systems, depending upon the preference of the surgeons practicing therein. Using the available systems of the prior art, the surgeon must preoperatively decide on the type of fixation treatment required and then chose an appropriate system to use. There has thus been a long felt need by those in this art for a single-system offering all the advantages of the prior systems, which may be adapted to a variety of differing operative conditions and applications, which also provides variable types of control at each level as needed.